The present invention relates to a logging tool for use with mud returns during the drilling of petroleum wells. More particularly, the present invention is directed to a method and logger apparatus for monitoring the helium isotope ratio of a gas sample taken from the cuttings-ladened, drilling fluid returns of a drilling system employing an oil-containing mud, so as to prevent unknowingly drilling through an oil-bearing formation.
In the drilling of oil and gas wells, two types of drilling fluids (or muds) are used: water-based and oil-based. While each type of fluid has its own set of advantages, the oil-based fluids are particularly useful in unconsolidated and water-susceptible formations. One problem with oil-based drilling fluids, however, is the possibility of drilling through an oil-bearing formation without knowing it, since the hydrocarbons in the drilling fluid will mask the formation fluids in the drilling mud returns, thus preventing visual identification. Even when water-based muds are used, diesel fuel or other middle to heavy hydrocarbons will typically be added to the mud system to help lubricate the drill bit causing a similar hydrocarbon-masking problem.
In taking a series of readings in pre-drilled wells, it was determined that the wells which were flowing best and represented the largest reservoirs had appreciably higher .sup.3 He to .sup.4 He ratios than did the depleted reservoirs and, more importantly, this helium isotope ratio was significantly higher in the liquid hydrocarbon deposits than in normal atmospheric conditions or even in the gaseous hydrocarbons. This discovery gave rise to the proposed method and apparatus of the present invention for avoiding overlooking drilling through an oil-bearing region.
The method of the present invention comprises taking a gas sample from the cuttings-ladened oil-containing drilling fluid returns, analyzing the sample to determine the amounts of .sup.3 He and .sup.4 He present, calculating the .sup.3 He/.sup.4 He isotope ratio, monitoring the magnitude of this helium isotope ratio as well as the levels of the two isotopes to be able to detect significant increases and/or other changes in these values which would indicate the presence or proximity of formation hydrocarbons and/or of significant structural variations.
Apparatus for performing the steps of this method comprises a gas trap positioned in the mud return line. As the cuttings-ladened mud returns pass through the gas trap, the helium isotopes, which had been held in solution in the liquid hydrocarbons by the downhole pressures, will be released and accumulate in the gas trap. A sampling nipple will permit gas samples to be extracted either continuously or periodically, as desired. At least a portion of the sample will be processed through a purification train to condense or precipitate out all gases from the sample which might interfere with a helium analysis. Another portion of the sample may be subjected to gas chromatography to analyze all hydrocarbon gases present and to provide a cross-check data point for the logging tool. The above-processed portion of the sample will be fed to a specially constructed mass spectrometer designed to examine these helium samples and to assess the .sup.3 He and .sup.4 He isotope components. The data output from this specialized mass spectrometer may be fed to (1) a data correlator/processor to be formatted for a computer, (2) to the computer directly if already in the proper format, (3) to a logger printer for tabulation with other data, (4) to a display screen, and/or 5) to an alarm/signal device to advise the operator that hydrocarbons are present. Other relevant data such as mud temperature, drill bit location and penetration rate, and hydrogen (and oxygen) levels in the drilling mud, may also be fed to the correlator and/or computer and plotted by the logger printer.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following detailed description.